The technical field of this invention is that of nondestructive materials characterization as applied to control of a process that changes the material properties. The nondestructive characterization provides a quantitative, model-based assessment of surface, near-surface, and bulk material condition for flat and curved parts or components. Characterization of bulk material condition includes (1) measurement of changes in material state, such as degradation/damage caused by fatigue damage, creep damage, thermal exposure, or plastic deformation; (2) assessment of residual stresses and applied loads; and (3) assessment of processing-related conditions, for example from component assembly or heat treatment. It also includes measurements characterizing material states, such as temperature. Characterization of surface and near-surface conditions includes measurements of surface roughness. Each of these includes detection of electromagnetic property changes associated with either microstructural and/or compositional changes, or electronic structure (e.g., Fermi surface) or magnetic structure (e.g., domain orientation) changes.
Manufacturing processes, such as a heat treatment or cold working, are commonly performed on materials in order to introduce beneficial characteristics, such as a desired hardness level or a residual stress distribution. Often, the beneficial characteristics, such as the residual stress from a cold work procedure, are hidden beneath the surface and spatially varying with depth into the material. Furthermore, these processes are often performed in a batch mode according to a preset processing schedule, with only an occasional post-process inspection or destructive evaluation performed to ensure material quality.
Similarly, many material systems and devices have multiple material layer or embedded materials and applying a process or operation of these systems can lead to material changes hidden from the surface. One example is for electronic materials, where temperature excursions due to power dissipation can lead to changes in the conductivity of the conducting pathways and inadvertent signal voltage decay (commonly called IR drop). This thermal or signal degradation is often predicted from simulation models so that it can be avoided during the build-up of the final product. It is typically not possible to embed thermal sensors themselves due to limited space or accessibility, and the state of the devices using these materials is usually obtained from the variations in the electrical signals.
Another example system is a solid oxide fuel cell (SOFC). SOFCs act as energy devices and usually contain stacks of individual cells connected together, with each cell containing a multiple layer system containing anode, electrolyte, cathode, and interconnect materials. The anode, cathode, and interconnect are electronic conductors while the electrolyte is an oxygen-ion conductor. SOFCs typically operate at relatively high temperatures near 1000° C. Advanced systems are highly instrumented and designed to obtain system level operational information such as stack and row voltages and currents, temperature, air and fuel gas composition and flow rates, system pressure and many other parameters. The overall health of such systems during operation is monitored mainly by combining information regarding stack and row voltages, currents and stack temperatures from various locations within the stack. While this methodology is very successful at obtaining a real-time health report of the SOFC power system, it provides little information on the health of individual cells.
Standard characterization techniques for layered electronic materials typically involve microstructural characterization and, in some cases, electrochemical characterization. Microstructural characterization, usually performed as part of a quality control process, typically involves resin-mounting sections of the materials using appropriate metallographic techniques followed by polishing and scanning electron microscopy. For example, electrode porosity is usually measured using the line intercept technique, by drawing several lines on the micrograph of the polished sections at equal angular intervals and measuring the fraction of the line inside pore space to the total length of the line. Other techniques of measuring pore volume fraction from polished cross sections of electrodes include using quantitative metallography techniques with macros that can be set up in software packages. These microstructural characterization techniques can provide comprehensive information about many features of the materials, including layer thicknesses, layer porosity, and adherence of layers. However, these techniques are destructive and slow, typically taking from seven to ten days to complete.
A common way to nondestructively characterize conducting materials is to use eddy-current methods. Conventional eddy-current sensing involves the excitation of a conducting winding, the primary, with an electric current source of prescribed frequency. This produces a time-varying magnetic field, which in turn is detected with a sensing winding, the secondary. The spatial distribution of the magnetic field and the field measured by the secondary is influenced by the proximity and physical properties (electrical conductivity and magnetic permeability) of nearby materials. When the sensor is intentionally placed in close proximity to a test material, the physical properties of the material can be deduced from measurements of the impedance between the primary and secondary windings. Traditionally, scanning of eddy-current sensors across the material surface is then used to detect flaws, such as cracks.
Existing magnetic/electromagnetic, diffraction, ultrasonic and other methods for assessment of residual stresses or monitoring of applied stress over wide areas are not yet practical or cost-effective. Typically, discrete strain gages are mounted directly onto the material under test (MUT). However this requires intimate fixed contact between the strain gage and the MUT and individual connections to each of the strain gages, both of which limit the potential usefulness for monitoring stress over large areas. Furthermore, strain gages are limited in durability and do not always provide sufficient warning of gage failure or malfunction. Correlations between magnetic properties and stresses in ferromagnetic materials have been studied for over 100 years. Magnetostriction effect data suggests that, depending on the magnitude and sign of the magnetostriction coefficient, correlation between stress and magnetic permeability within certain ranges of the magnetic field should be present. However, attempts to use conventional inductive, i.e., eddy-current, sensors for assessment of residual stresses as well as for a number of other applications have shown significant limitations, particularly for complex geometry components.